1. Field of the Invention
This invention relates to an electrophotographic photosensitive member provided with an intermediate layer between the substrate and the photosensitive layer, and particularly to a novel intermediate layer with which performance characteristics of the electrophotographic photosensitive member have been improved.
2. Description of the Prior Art
Electrophotographic photosensitive members have on a substrate a photosensitive layer containing a photoconductive material such as Se, Se-Te, Se-Te-As, ZnO, CdS, CdSe, amorphous Si, PVK, phthalocyanine, pyrazoline, or the like.
Organic photoconductive materials have many advantages over inorganic ones, such as the lightness in weight, high productivity, and so forth, but were difficult to put into practical use, on account of the low sensitivity. In consequence, some sensitizing techniques have been suggested, among which an effective one is to use a laminate type photosensitive member having two laminated photosensitive layers which function separately as a charge generation layer and a charge transport layer.
In this type of photosensitive member, the charge generation layer is very thin and hence is liable to be affected by the surface state of the substrate when formed thereupon, that is, the roughness of the substrate surface causes coating irregularities or thickness irregularities of the charge generation layer. Occurrence of these irregularities must be prevented as far as possible because they will cause defects or uneven density in the resulting images. Therefore the substrate surface is finished in the conventional process to reduce the maximum surface roughness to 1.mu. or less by additional processing such as grinding or polishing. Such processing results in the raise of the cost of electrophotographic photosensitive members.
Thus, an intermediate layer has become set up between the substrate and the photosensitive layer aiming at eliminating these irregularities and achieving other effects such as improvements of the adhesion and coating workability of the photosensitive layer material, protection of the substrate, covering of surface defects of the substrate, protection of the photosensitive layer from electric destruction, and improvement of the injectability of electric charge from the substrate into the photosensitive layer.
It is known that the intermediate layer can be formed from, e.g. poly(vinyl alcohol), poly(vinyl methyl ether), poly(N-vinylimidazole), ethyl cellulose, methyl cellulose, ethylene-acrylic acid copolymer, casein, gelatin, or polyamide.
Characteristics required to the intermediate layer are, in the first place, electric properties. Since this layer is used in an electrophotographic photosensitive member, it is important for the layer to exert no adverse effect on the electrophotographic characteristics of the photosensitive member and hence to have a low electric resistance. If the electric resistance is too high, some of the charged potential is also applied to the intermediate layer and remains as so-called residual potential, which will cause the fogging of formed image.
In addition, the electric resistance of the intermediate layer is required not to be affected by any change in the ambient conditions, particularly by the variation of the atmospheric humidity. For instance, if the electric resistance increases with decreasing humidity, the fogging will be caused by low humidity.
While such characteristics as noted above are required by the intermediate layer, these requirements were scarcely satisfied in the past with a single-resin layer. Thus, according to the prior art, a very thin resin layer or a resin layer in which a conductive powder (powder of nickel, copper, silver, or some other metals) is dispersed is used as an intermediate layer. However, decreasing the thickness of the resin layer is defective since this sacrifies functions of the intermediate layer, and the metallic powder dispersion layer is also defective in that the surface property of the intermediate layer is impaired by coarse metallic particles.